Add rule to identify orphaned channel components

[jonaslagoni]

User story.
As a user, I can detect if my document has orphaned channels inside components.

Describe the solution you'd like
This should be a valid AsyncAPI file:

{
  asyncapi: '2.3.0',
  channels: {
    'users/signedUp': {
      $ref: '#/components/channels
    },
  },
  components: {
    channels: {
      userSignedUp: {
        subscribe: {
          message: {
            payload: {
              type: 'string',
            },
          },
        },
      },
    },
  },
}

The new ruleset should catch this invalid case:

{
  asyncapi: '2.3.0',
  channels: {},
  components: {
    channels: {
      userSignedUp: {
        subscribe: {
          message: {
            payload: {
              type: 'string',
            },
          },
        },
      },
    },
  },
}

In order to solve this you need to do the following steps:

1. Add the new rule to the AsyncAPI ruleset: https://github.com/stoplightio/spectral/blob/develop/packages/rulesets/src/asyncapi/index.ts
2. Add a test for the new rule: https://github.com/stoplightio/spectral/blob/develop/packages/rulesets/src/asyncapi/__tests__/asyncapi-unused-components-schema.test.ts
3. Add the rule to the documentation: https://github.com/stoplightio/spectral/docs/reference/asyncapi-rules.md

Additional context
Part of #1103
Related PR that can be used as an example: #2097

[jonaslagoni]

Closing as we create one rule for all instead, as OpenAPI has: #1103 (comment)